MBE is a well known process which is used to produce semiconductor devices, e.g. devices for optical telecommunications such as lasers, detectors and optical amplifiers.
The MBE process comprises projecting a molecular beam of reactant species at the surface of a substrate. The process is carried out at low pressure such that the length of the beam is considerably less than the mean free path of the molecules. Under these conditions there are no collisions in the beam and hence all the reaction takes place on the surface of the substrate. The very low pressure at which MBE operates will sometimes be called "vacuum".
MBE is used to deposit many different compound chemical species, comprised of many elements from many of the groups in the periodic table. One important application is the use of MBE to produce semiconductor devices made of compounds formed from a combination of elements from groups III and V of the periodic table. Examples of III/V compound semiconductors produced by MBE include GaAs, InP and AlAs with dopants such as Be and Si.
Most existing MBE systems evaporate the required components of a deposited layer from individual thermal evaporation cells containing the material in elemental form. These are effective, but have a number of disadvantages. For example, they need constant calibration and "spitting" from the cells gives rise to morphological defects in the deposited layers. A further problem is that effusion cells have a finite lifetime before they need refilling. This involves venting the MBE system to atmosphere which is time consuming and has deleterious effects on system calibration and layer morphology. Many of these problems have recently been alleviated by the replacement of the effusion cells with gas sources. One of the main advantages with gas sources is that they can be replenished without the need to vent the main system. Thus the group V elements are introduced to the MBE system as the gaseous hydrides (e.g. AsH.sub.3 and PH.sub.3), using a specially designed gas handling system, and the group III and dopant elements as metal organic compounds (MOC's), e.g. In(CH.sub.3).sub.3, Ga(C.sub.2 H.sub.5).sub.3 and Al(CH.sub.3).sub.3 plus Be(C.sub.2 H.sub.5).sub.2 and Si(C.sub.2 H.sub.5).sub.4 for dopants.
The MOC's are all volatile liquids or solids which have been widely used in other epitaxial processes, e.g. metal organic chemical vapour deposition. Thus equipment to handle these compounds is already available, e.g. flasks which include stopper-valves and connectors for attaching valves to growth apparatus.
Proposed methods of using metal organic compounds have not been entirely satisfactory for MBE. For example mass flow controllers do not work well at the low flow rates needed for MBE. In addition the mass flow controllers must be situated outside the reaction chamber and this gives rise to difficulties.
Direct evaporation into an antechamber has also been proposed. The antechamber is connected to the reaction chamber via a capilliary tube which is provided with an on/off valve to switch reactant between layers.
The necessary control has included a feedback loop to stabilise the pressure in the antechamber. Although this method works it is complex and has slow switching times.
Thus the problem exists of obtaining a beam with better uniformity than the beam provided by effusion cells with a source that provides fast switching times.
This invention proposes a solution in which the saturated vapour of a metal organic compound passes into a mixer manifold via an on/off control valve and a resistance valve. The mixer manifold is effectively part of the MBE reaction chamber. In preferred embodiments the mixer manifold produces a uniform beam and this may obviate the need for a rotating substrate.
The high resistance valve constitutes the interface between the vacuum part of the reactor and the reservoir flask which holds the reserve of metal organics. The flow rate is determined by the pressure drop across the resistance which is substantially constant. Since the low pressure is substantially zero the high pressure constitutes the primary control. Maintaining a liquid metal organic compound in thermal equilibrium with its vapour constitutes an effective way of adjusting this primary control.
As stated above, the mixer manifold gives rise to a uniform beam which is partially collimated. It should be noted that a collimating manifold can be used with sources other than the combinations of resistance valve/on-off valve and thermostat mentioned above. The collimating manifold can also be used with mass flow controllers, pressure regulated sources and low pressure vapour source mass flow controllers.
The collimating manifold comprises:
(i) An exit port adapted for connection to supply a collimated beam to an MBE reactant chamber; PA1 (ii) A reception zone wherein inlet ports for the admission of reagents are located; and PA1 (iii) An elongate collimation zone situated between the reception zone and the exit port.
The collimation zone has two important functions, namely to produce a beam which is sufficiently uniform in respect of both composition and flux. This is facilitated when the ratio: ##EQU1## is greater than 10, preferably greater than 50.
It will be apparent that the cross sectional area of the beam should be large enough for the beam to cover the whole of the substrate. The substrate is often circular and, therefore, a collimation zone with a circular cross section is appropriate.
In addition to uniformity, it is also important to achieve rapid switching in order to grow a device with layers having sharp interfaces. This requires that, when "switching off", molecules which remain in the collimation zone dissipate rapidly and, therefore, a low flow resistance, e.g. with no small dimensions, is appropriate. It has already been mentioned that a circular cross section is appropriate and this configuration is also suitable to give a low flow resistance.
Thus a cylindrical collimation zone, preferably having an axial length at least three times its diameter, is particularly appropriate.
For use with a substrate having a diameter of 5 cm, a cylindrical collimation zone with a diameter of 6 cm and a length 50 cm would be suitable. For such a collimation zone the parameters quoted above have the following values:
______________________________________ LENGTH 50 cm DIAMETER 6 cm LENGTH:DIAMETER 8.3 VOLUME 1.4 liters R 88 ______________________________________
The collimating manifold may also incorporate a secondary source for other reagents, e.g. reagents such as PH.sub.3 and AsH.sub.3 which require thermal cracking. It may be desirable to release these reagents at the exit port and to locate the thermal cracker at the exit port. The secondary source conveniently takes the form of a supply tube, adapted to convey a mixture of secondary reagents, situated on the axis of the collimation zone. A thermal cracker is included as part of the secondary source.